The present invention relates to an optical imaging device. The invention may be used in the context of microlithography used for fabricating microelectronic circuits. Thus, it further relates to an optical imaging method which, among others, may be implemented using such an optical imaging device.
Especially in the area of microlithography, apart from the use of components having a high precision, it is necessary to keep the position and the geometry of the components of the imaging device, e.g. the optical elements such as lenses, mirrors and gratings, unchanged during operation to the highest possible extent in order to achieve a correspondingly high imaging quality. The tough requirements with respect to accuracy (lying in the magnitude of a few nanometers or below) are none the less a consequence of the permanent need to reduce the resolution of the optical systems used in fabricating microelectronic circuitry in order to push forward miniaturization of the microelectronic circuitry to be produced.
In order to achieve an increased resolution either the wavelength of light used may be reduced as it is the case with systems working in the extreme UV (EUV) range at working wavelengths in the area of 13 nm or the numerical aperture of the projection system used may be increased. One possibility to remarkably increase the numerical aperture above the value 1 is realized in so-called immersion systems, wherein an immersion medium having a refractive index larger than 1 is typically placed between the last optical element of the projection system and the substrate to be exposed. A further increase in the numerical aperture is possible with optical elements having a particularly high refractive index.
It will be appreciated that, in a so-called single immersion system, the immersion element (i.e. the optical element at least in part contacting the immersion medium in the immersed state) typically is the last optical element located closest to the substrate to be exposed. Here, the immersion medium typically contacts this last optical element and the substrate. In a so-called double immersion system, the immersion element does not necessarily have to be the last optical element, i.e. the optical element located closest to the substrate. In such double or multiple immersion systems, and immersion element may also be separated from the substrate by one or more further optical elements. In this case, the immersion medium the immersion element is at least partly immersed in may be placed, for example, between two optical elements of the optical system.
With the reduction of the working wavelength as well as with the increase of the numerical aperture not only the requirements with respect to the positioning accuracy and the dimensional accuracy of the optical elements used become more strict throughout the entire operation. Of course, the requirements with respect to the minimization of imaging errors of the entire optical arrangement increase as well.
The deformations of the respective optical element and the imaging errors resulting therefrom are of special importance in this context. Eventually, even the own weight of the optical elements leads to a non-tolerable deformation. In order to counteract these deformations induced by the own weight it is proposed in U.S. Pat. No. 6,243,159 B1 (Nakao) and in U.S. Pat. No. 6,388,731 B1 (Nakao)—the entire disclosure of each of which is enclosed herein by reference—to have atmospheres of different pressure acting on both sides on the optical element in such a manner that the pressure difference generates a counterforce corresponding to the gravitational force and acting on the optical element.
By this means it may be possible to achieve a high imaging quality, i.e. low imaging errors, in a stationary state. However, non-stationary factors leading to dynamic pressure variations in the atmosphere acting on the optical element still cause problems. Depending on the rigidity of the holder of the optical element such pressure variations may result in a shift of the respective optical element with respect to the remaining components of the imaging device leading to non-negligible imaging errors.
In order to counteract such pressure variation induced imaging errors with an optical element contacted by two different atmospheres U.S. Pat. No. 5,636,000 (Ushida et al.)—the entire disclosure of which is incorporated herein by reference—proposes to have adjustable optical correcting elements such as gratings or the like within the imaging device. Compensation of captured imaging errors as provided in this imaging device by adjusting the position of these optical correcting elements.
This solution has the disadvantage that it causes considerable expense for its development since the optical correction elements have to be considered in the outlay of the optical system and have to be integrated therein. Furthermore, typically, considerable expense is required for actuation of the optical correcting elements. Finally, eventually, the optical correcting elements themselves are a subject to the pressure variation such that the expense for the correction is increased.